This invention relates to thermal barrier coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a method for forming a thermal barrier coating system that exhibits improved resistance to spalling and low thermal conductivity.
Higher operating temperatures of gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys, though such alloys alone are often inadequate to form components located in the hot sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to thermally insulate such components in order to minimize their service temperatures. For this purpose, thermal barrier coatings (TBCs) formed on the exposed surfaces of high temperature components have found wide use.
To be effective, TBCs must have low thermal conductivity, be capable of being strongly adhered to the article, and remain adherent through many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between materials having low thermal conductivity and superalloy materials used to form turbine engine components. For this reason, TBCs have generally employed a metallic bond coat deposited on the surface of a superalloy component. A thermal-insulating layer (the TBC) is then deposited on the bond coat, which together form what is termed a TBC system. The metallic bond coat is typically a diffusion aluminide or an oxidation-resistant alloy, such as MCrAlY where M is iron, cobalt and/or nickel, which promotes the adhesion of the insulating layer to the component while also inhibiting oxidation of the underlying superalloy.
Various ceramic materials have been employed as the TBC, particularly zirconia (ZrO2) stabilized by yttria (Y2O3), magnesia (MgO) or other oxides. These particular materials are widely employed in the art because they can be readily deposited by plasma spray, flame spray and vapor deposition techniques. A continuing challenge of TBC systems has been the formation of a more adherent ceramic layer that is less susceptible to spalling when subjected to thermal cycling. For this purpose, the prior art has proposed various coating systems, with considerable emphasis on ceramic layers having enhanced strain tolerance as a result of the presence of porosity, microcracks and segmentation of the ceramic layer. Segmentation indicates that the ceramic layer has columnar grain boundaries or cracks oriented perpendicular to the surface of the component, and is achieved with electron beam physical vapor deposition (EBPVD) techniques. As is known in the art, a TBC having a columnar grain structure is able to expand with its underlying substrate without causing damaging stresses that lead to spallation, as evidenced by thermal cyclic testing.
Zirconia-based TBCs, and particularly yttria-stabilized zirconia (YSZ) coatings having columnar grain structures and a thickness on the order of about 125 micrometers (about 0.005 inch) or more are widely employed in the art for their desirable thermal and adhesion characteristics. Processes for producing YSZ coatings with EBPVD techniques generally entail suspending a component in a coating chamber above one or more YSZ ingots, and then melting a surface of the ingot(s) with an electron beam to generate a vapor of zirconium, yttrium and oxygen ions and nonstoichiometric metal oxides that recombine to form a YSZ coating on the component surface. The deposition rate and the throughput of components are generally limited by the size of the coating zone within the coating chamber. As used herein, the xe2x80x9ccoating zonexe2x80x9d is the volume within the coating chamber over which an allowable variation of deposition rates is established, usually within about xc2x110%. The deposition rate can be taken as the average deposition rate over the entire coating zone.
The coating chamber is conventionally evacuated and then backfilled with oxygen or a mixture of oxygen and an inert gas (typically argon) to maintain a pressure of 0.005 mbar or less during the coating operation. When depositing a TBC of a metal oxide such as YSZ, the addition of oxygen is for the purpose of providing excess oxygen ions, which ensures that near-stoichiometric oxides (e.g., zirconia and yttria) are formed. Low chamber pressures are employed to avoid adversely affecting the operational characteristics of the electron beam gun and the baffling chambers used therewith. More particularly, higher pressures are avoided because control of the electron beam is more difficult at pressures above about 0.005 mbar, with erratic operation being reported at coating chamber pressures above 0.010 mbar. It has also been believed that the life of the gun filament would be reduced or the gun contaminated if operated in pressures above 0.005 mbar.
While greater coating pressures have been reported for EBPVD operations, such as in U.S. Pat. Nos. 4,006,268 to Kennedy et al., U.S. Pat. No. 5,645,893 to Rickerby et al., and U.S. Pat. No. 5,716,720 to Murphy, the 0.005 mbar upper pressure limit has been largely adhered to by those skilled in the art when depositing ceramic TBCs. Notably, Kennedy et al. are concerned with depositing metal coatings, and teach that higher pressures collimate the vapor produced when evaporating a single metal ingot by EBPVD. Therefore, according to Kennedy et al., higher pressures yield a more focused path between the ingot and the targeted surface, which is said to increase the efficiency of the coating process. However, the uniform coating zone resulting from Kennedy""s process is limited in size by the collimated vapor. Also of note is that, while Rickerby et al. and Murphy are concerned with depositing YSZ coatings, Rickerby et al. limit their pressures of 0.013 mbar, Murphy uses only oxygen as the coating atmosphere and, as with Kennedy et al., both are limited to evaporating a single ingot.
Though YSZ deposited by EBPVD is a highly successful coating system for protecting turbine engine components, there is an ongoing effort to improve the spallation resistance and reduce the thermal conductivity of TBCs, and to deposit such coatings by more efficient processes.
The present invention is a method for producing a thermal barrier coating system on an article which will be subjected to a hostile environment that promotes spallation, as is the case with turbine, combustor and augmentor components of a gas turbine engine. The coating system is composed of a metallic bond coat and a thermal barrier coating (TBC) having a columnar grain structure. According to the invention, the bond coat is prepared and the TBC subsequently deposited by electron beam physical vapor deposition (EBPVD) by an improved coating process whose parameters significantly improve spallation resistance and reduce the thermal conductivity of the TBC.
The method of this invention generally entails establishing an absolute pressure of at least 0.014 mbar within a coating chamber with a gas (or gas mixture) that preferably includes oxygen and an inert gas, though an oxygen-free coating atmosphere is also within the scope of this invention. A metal oxide ceramic material is then evaporated with electron beams focused on at least two masses (e.g., ingots) of the ceramic material so as to produce a vapor of metal ions, oxygen ions and one or more nonstoichiometric metal oxides. According to the invention, the vapor is diffuse relative to the ingots of ceramic material as a result of the elevated pressure, and travels upwardly and outwardly from the mass toward a component above the mass within the coating chamber. The metal and oxygen ions and nonstoichiometric metal oxides recombine to form a layer of the ceramic material on the surface.
Contrary to Kennedy et al., under these conditions the ceramic material evaporates to form an ion vapor that is not collimated, but instead is diffuse relative to the ceramic ingots, allowing for more parts to be simultaneously coated. A metal oxide TBC deposited in accordance with this invention is porous and tenaciously adheres to a bond coat, and surprisingly is significantly more spall resistant than TBCs deposited under conventional conditions, i.e., lower chamber pressures and a single ingot of ceramic material. Also unexpected is that the coating deposition rate is dependent on chamber pressure, with higher pressures corresponding to higher deposition rates and more efficient usage of ceramic material, i.e., more ceramic deposited on a turbine component per unit length of ingot consumed (volume evaporated). Accordingly, the method of the present invention not only improves the spallation resistance of the resulting TBC, but also improves manufacturing economies.
Other objects and advantages of this invention will be better appreciated from the following detailed description.